Antennas of a wireless communication system are often classified as either a virtual antenna called an antenna port, or a physical antenna for actually sending and receiving signals. In general, the virtual antenna and the physical antenna of the wireless communication system correspond to each other by 1:1.
To detect a signal, a receiving stage estimates a channel between the virtual antenna of a transmitting stage and the antenna of the receiving stage, and detects a size of the transmit signal and phase modulation using the channel. The transmitting stage sends a reference signal per virtual antenna so that the receiving stage can estimate the channel of the virtual antenna.
The reference signal is allocated for the receiving stage to estimate the channel to the transmitting stage. To reject interference from a traffic channel, a resource is allocated separately from the traffic channel. For example, when the transmitting stage includes four virtual antennas, it allocates the reference signal to each virtual antenna as shown in FIG. 1.
FIG. 1 depicts the resources of the reference signal in a conventional multi-antenna system. Hereafter, it is assumed that the multi-antenna system employs an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
When the transmitting stage includes four virtual antennas as shown in FIG. 1, it sends the reference signal over 0-th, fourth, seventh and eleventh OFDM symbols of the 0-th virtual antenna and the first virtual antenna. The transmitting stage sends the references signal over the first and eighth OFDM symbols of the second virtual antenna and the third virtual antenna. That is, the transmitting stage allocates the resources of the reference signal to send the reference signal over the same OFDM symbols of the 0-th virtual antenna and the first virtual antenna and to send the reference signal over the same OFDM symbols of the second virtual antenna and the third virtual antenna.
As sending the reference signal, the transmitting stage does not transmit any signal at the same location as the reference signal transmission location of the different virtual antennas in the resources of the virtual antennas.
As sending the reference signal per virtual antenna, the transmitting stage sends the reference signal with a transmit power four times the transmit power of the traffic per resource block. In the tone carrying the traffic, except for the reference signal, every virtual antenna of the transmitting stage sends the signal with the same transmit power.
Accordingly, the power used by the symbol carrying the reference signal per virtual antenna varies per virtual antenna. For example, when the transmit power of the traffic is set to A, over the 0-th, fourth, seventh, and eleventh OFDM symbols in FIG. 1, the 0-th virtual antenna and the first virtual antenna consume the power of 4 A and the second virtual antenna and the third virtual antenna consume the power of 2 A.
Further, when the virtual antennas and the physical antennas correspond to each other by 1:1, the power imbalance of the virtual antennas causes imbalance in the power of the physical antennas. As a result, each physical antenna needs to be designed on different bases.